Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Nanostructures for Enhanced Glycerol Photooxidation

RESEARCH ARTICLE
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Isolated Iridium Sites on Potassium-Doped Carbon-nitride
wrapped Tellurium Nanostructures for Enhanced Glycerol
Photooxidation
Pawan Kumar, Abdelrahman Askar, Jiu Wang, Soumyabrata Roy,
Srinivasu Kancharlapalli, Xiyang Wang, Varad Joshi, Hangtian Hu, Karthick Kannimuthu,
Dhwanil Trivedi, Praveen Bollini, Yimin A. Wu, Pulickel M. Ajayan, Michael M. Adachi,*
Jinguang Hu,* and Md Golam Kibria*
Many industrial processes such transesterification of fatty acid for biodiesel
production, soap manufacturing and biosynthesis of ethanol generate glycerol
as a major by-product that can be used to produce commodity chemicals.
Photocatalytic transformation of glycerol is an enticing approach that can
exclude the need of harsh oxidants and extraneous thermal energy. However,
the product yield and selectivity remain poor due to low absorption and
unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te)
nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride
(KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance
charge separation and visible-NIR absorption. The iridium (Ir) single atom
sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes
selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6%
and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity
can reach up to 88% under 450 nm monochromatic light. X-ray absorption
spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2
sites which improved catalytic selectivity for glycol oxidation. Band energies
and computational calculations reveal faile charge transfer in the TeKCNIr
heterostructure. EPR and scavenger tests discern that superoxide (O2
•−
) and
hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
P. Kumar, J. Wang, H. Hu, K. Kannimuthu, D. Trivedi, J. Hu, M. G. Kibria
Department of Chemical and Petroleum Engineering
University of Calgary
2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada
E-mail: jinguang.hu@ucalgary.ca;md.kibria@ucalgary.ca
A.Askar,M.M.Adachi
School of Engineering Science
Simon FraserUniversity
Burnaby,BC V5A 1S6,Canada
E-mail: mmadachi@sfu.ca
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202313793
© 2024 The Authors. Advanced Functional Materials published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.
DOI: 10.1002/adfm.202313793
1. Introduction
To tackle the challenges of rising petroleum
prices and CO2 concentration, govern-
ments around the globe are enacting
“biofuel blending mandates” which ne-
cessitated the addition of 2–10% biofuel
in petroleum fuel.[1]
Biodiesel production
from the transesterification of fatty acid oc-
cupies a market size worth USD 32.09 bil-
lion and is expected to grow at the rate of
10.0% from 2022 to 2030.[2]
For each 10
Kg biodiesel production from the transes-
terification process, 1 Kg glycerol is pro-
duced as a major by-product which has lim-
ited utility in the cosmetic, food, tobacco
and pharmaceutical industries.[3]
Utilizing
the carbon content of glycerol to upgrade to
value-added chemicals is a valuable comple-
ment. Many chemocatalytic processes such
as hydrogenolysis (1,2 propanediol, ethy-
lene glycol), dehydration (acrolein, acetol),
oxidation (dihydroxyacetone; DHA, glyc-
eric acid, hydroxypyruvic acid, formic acid),
S. Roy, P. M. Ajayan
Department of Materials Science and Nanoengineering
Rice University
Houston, TX 77005, USA
S. Kancharlapalli
Theoretical Chemistry Section
Chemistry Division
Bhabha Atomic Research Centre
Trombay, Mumbai 400085, India
S. Kancharlapalli
Homi Bhabha National Institute
Anushaktinagar, Mumbai 400094, India
X. Wang, Y. A. Wu
Department of Mechanical and Mechatronics Engineering
Waterloo Institute for Nanotechnology
Materials Interface Foundry
University of Waterloo
Waterloo, Ontario N2L 3G1, Canada
Adv. Funct. Mater. 2024, 2313793 2313793 (1 of 15) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH

www.advancedsciencenews.com www.afm-journal.de
amination (alanine) etc have been investigated to upgrade the
glycerol.[4]
Among them, catalytic oxidation of glycerol is more
conducive to scale-up due to the requirement of a simple oxida-
tion process. Photocatalytic oxidation of glycerol can proceed in
benign conditions and exclude the requirement of harsh chem-
icals and extraneous temperature.[5]
However, the oxidation of
glycerol is a complex reaction pathway and can accumulate sev-
eral oxidation products in the reaction mixture which reduces
economic value and adds purification cost. Therefore, develop-
ing photocatalysts with improved chemoselectivity is essential to
make the process profitable. 2D graphitic carbon nitride (CN; g-
C3N4) with an N-linked heptazine (C6N7) network has emerged
as a material of choice due to its desirable photophysical prop-
erties, chemoselectivity and tunable optical band gap.[6]
The op-
tical absorption of CN can be further red-shifted to a longer
wavelength region by cyanamide (N-C = N; CN2) functional-
ization and alkali metal doping.[7]
Lotsch et al. demonstrated
that post-annealing of melon with KSCN affords CN2 function-
alized carbon nitride which can significantly boost the hydro-
gen evolution.[8]
Later Yu et al. demonstrated that the presence
of alkali (KOH) during thermal annealing cleaves C6N7 and in-
troduces CN2 nitrogen defect which downshifts the LUMO posi-
tion to improve the visible light-driven hydrogen evolution.[9]
De-
spite the improved absorption, the catalytic performance of CN
remains poor due to the severe charge recombination. Further-
more, due to the less positive valence band (VB) position of CN
(+1.6 eV vs NHE), the hole migration in CN remains poor which
compromises its oxidizing power.[10]
Fabrication of heterojunc-
tion between modified carbon nitride with low bandgap semicon-
ductors has been envisioned to improve the charge separation
kinetics. Recently discovered 2D tellurene (Te) possesses a low
band gap (0.92 eV for monolayer; 0.33 eV for bulk) and p-type na-
ture with a hole mobility of ≈700 cm2
V−1
s−1
(3 times higher than
black phosphorous; black P) is an ideal candidate to make hetero-
junction with poor oxidizing n-type semiconductors.[11]
Qiu et al.
designed a heterojunction between CdS and Te which can afford
a better charge separation resulting in efficient oxidation of lactic
acid to hydrogen.[12]
CN can form vdW heterojunction with Te
which excludes the requirement of epitaxial matching and min-
imizes the carrier loss at semiconductor interfaces.[13]
The sur-
face oxidation of Te also forms native tellurium dioxide (TeO2)
(2.29 eV) with high theoretical electron (1000 cm2
V−1
s−1
) and
hole (9100 cm2
V−1
s−1
) mobility which can further reinforce
charge separation.[14]
In addition to improved visible absorption and charge separa-
tion, the product selectivity is largely governed by the nature of
catalytic centers. In bulk materials, the unsymmetrical exposed
facets and multiple binding sites lead to non-selective reactions.
Atomic scale distribution in single atom catalysts (SACs) can
minimize non-uniform active sites while an undersaturated coor-
dination site increases the rate of reaction.[15]
Due to their high
stability and requirement of a diluted concentration SACs can
V. Joshi, P. Bollini
William A. Brookshire Department of Chemical and Biomolecular
Engineering
University of Houston
Houston, TX 77204, USA
significantly reduce the operation cost. Various SACs decorated
on inorganic crystalline scaffolds have been used for selective
photocatalytic glycerol conversion.[16]
Xiong et al. showed that Ni
single atom grafted TiO2 (Ni/TiO2) can achieve a 60% selectivity
for glycerol to glyceraldehyde conversion.[17]
However, the pres-
ence of nonuniform facets, vacancies and tedious synthetic pro-
cess with a low single atom (SA) site population limits catalytic
activity/selectivity of inorganic support grafted SACs.[18]
On the
other hand, CN-based supports provide uniform C6N7 cavities
and structural flexibility to accommodate the isolated sites effec-
tivity for improved catalytic performance of the SACs.[19]
Care-
ful selection of the SA site is also crucial to get the desired cat-
alytic effect. Chen et al. demonstrated that low-coordinate IrO2
nanoparticles (NPs) decorated on CN can significantly increase
the oxygen evolution reaction.[20]
Motivated by these findings, herein, we design K-doped/CN2
functionalized CN (KCN) and Te-nanostructures based hetero-
junction followed by decoration of IrN2O2 isolated sites (TeKC-
NIr) for glycerol oxidation. The TeKCNIr catalyst demonstrated
superior H2 evolution performance and glycerol oxidation to glyc-
eraldehyde. After 24 h reaction under AM1.5G irradiation, TeKC-
NIr demonstrated a glycerol conversion of 45.6% with a catalytic
selectivity of 61.6%. Under optimized conditions and 450 nm vis-
ible light irradiation, an astonishing 88.6% selectivity for the glyc-
eraldehyde was achieved. The well-distributed IrN2O2 catalytic
sites as revealed by XAS were attributed to drive selective catalytic
oxidation of glycerol.
2. Results and Discussions
2.1. Synthesis and Characterization
The synthesis of various catalytic components is depicted in
Figure 1a and Figures S1,S2 (Supporting Information). The
potassium-doped CN2-functionalized carbon nitride (KCN) was
synthesized by thermal annealing of dicyandiamide and potas-
sium thiocyanate at 550 °C for 2 h (Figure 1a and section 2.1 in
Supporting Information).[21]
While the mixture of elemental 1D
Te nanorods (NRs) and 2D Te nanosheets (NSs) was prepared
by the hydrothermal reduction of sodium tellurite (Na2TeO3) us-
ing hydrazine (NH2NH2) in the presence of polyvinylpyrrolidone
(PVP) as a crystal face-blocking ligand (section 2.2 in SI).[11b]
The
KCN-wrapped Te NRs/NSs (TeKCN) vdW nanostructures were
prepared by taking advantage of electrostatic interaction between
negatively charged KCN and positively charged PVP function-
alized Te NRs/TeNSs (Figure 1b and section 2.3 in Supporting
Information). Three samples containing 5, 10, and 15wt% Te
(denoted as TeKCN5, TeKCN10 and TeKCN15) were prepared
to evaluate the catalytic performance. Since TeKCN10 demon-
strated the highest photocatalytic performance, it was used for
single atom decoration and TeKCN will be generally referred to
as TeKCN10 catalyst. The Ir single atom sites were deposited on
TeKCN catalysts by hydrolysis decomposition of IrCl3.xH2O us-
ing a very diluted aqueous solution (Figure 1c and section 2.4 in
For comparison, IrSA pinned KCN
(KCNIr) and CN (CNIr) were also synthesized (Figure S1a,b, sec-
tions 2.5 and 2.6 in Supporting Information). A control sample
(TeKCNIrP) by mixing Te NRs/NSs and KCN in DMF solvent fol-
lowed by the immediate addition of IrCl3.xH2O was also prepared
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Figure 1. a) Synthesis scheme of K doped and CN2-functionalized carbon nitride (KCN) b) Te NRs/NSs wrapping with KCN c) Decoration of Ir single
atoms on TeKCN to fabricate TeKCNIr.
(Figure S1c, section 2.7 in Supporting Information). Pristine CN
was prepared by polymerization condensation of dicyandiamide
(Figure S2, section 2.8 in Supporting Information).[23]
The fine chemical attributes of materials were determined us-
ing high-resolution transmission electron microscopy (HR-TEM)
(Figure 2; Figures S3–S14, Supporting Information). The TEM
image of CN displayed graphenic 2D sheets with an amorphous
structure (Figure S3, Supporting Information). KCN also dis-
played a similar 2D structure, however, it showed slightly in-
creased crystallinity as evident from the FFT and SAED pattern
(Figure S4a–d, Supporting Information). The EDX elemental
mapping of KCN displayed the presence of ample K distributed
uniformly in the material (Figure S4e–k, Supporting Informa-
tion). TEM images of TeNRs/KCN show a wrapped 1D nanorod
structure with a dense Te core and light KCN shell (Figure S5a–c,
Supporting Information). The thickness of the core was mea-
sured to be ≈ 420 nm while the KCN shell was ≈ 5–10 nm (Figure
S5c, Supporting Information). Elemental mapping of TeKCN dis-
played a significant contribution of O signal in elemental map-
ping due to surface oxidation of Te into TeO2 which was also
confirmed from X-ray photoelectron spectroscopy (XPS) analy-
sis (Figure S5e–l, Supporting Information). Furthermore, strong
K and N signals overlapping with Te, and O signals verify the
uniform coating of KCN in the materials (Figure S5e–l, Sup-
porting Information). The images of TeKCN at high magnifica-
tion show a continuous lattice fringe of 2.5 Å spacing as evident
from FFT and iFFT suggesting a monocrystalline core embed-
ded in the KCN structure (Figure S6, Supporting Information).
TeNSs/KCN also displayed a similar pattern with the uniform
wrapping of KCN all over the surface (Figure S7, Supporting In-
formation). After the decoration of Ir SA sites in TeKCNIr, the
dark Te core and KCN shell morphology of TeKCN remains un-
changed (Figure 2a,b). The 5–10 nm thick KCN shell was mostly
amorphous, and observance of some lattice fringes was due to
the stacking of CN sheets (Figure 2c,d) TeKCNIr composed of
TeNSs showed flat Te sheets with the comparatively thin wrap-
ping of KCN (Figure 2e,f). The filtering of different d-spacing
signals using iFFT reveals the presence of three planes in the ma-
terials with a d-spacing of 1.9, 2.6 and 4.0 Å (Figure 2g).[24]
The
selected area electron diffraction (SAED) shows a reciprocal plane
and represents preserved Te crystallinity.[25]
The EDX elemental
mapping displayed a uniform distribution of Te, O, K, C and N
suggesting uniform wrapping, however, Ir was not visible due
to lower concentration (Figure 2i–t). Electron energy loss spectra
(EELS) of TeKCNIr exhibited energy loss peaks for C K-edge, N
K-edge, O K-edge and Te M-edge. However, the energy loss peak
for Ir cannot be detected due to the diluted concentration of Ir
(Figure S8, Supporting Information). TEM image of KCN with
IrSA sites (KCNIr) displayed graphenic sheets with a lattice spac-
ing of 3.3 Å due to improved crystallinity while no Ir/Ir-oxide re-
lated features were observed suggesting the atomic distribution
of Ir sites (Figure S9, Supporting Information). The AC-HAADF-
STEM image clearly showed the presence of dense Ir sites pinned
on sparse CN sites (Figure S10, Supporting Information). Control
TeKCNIrP catalysts showed spherical Te nanoparticles embed-
ded in KCN sheets due to the breakdown of Te nanostructures
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Figure 2. HR-TEM images of TeKCNIr NRs a) at 100 nm scale bar showing dense Te-core or thin KCN shell. b) at 20 nm scale bar c,d) at 5 nm scale bar
showing the presence of dense core and sparse amorphous KCN shell. HR-TEM images of TeKCNIr nanosheets e) at 0.2 μm scale f) at 5 nm scale showing
dominant Te single crystal lattice and amorphous KCN at the edge. Inset showing FFT of image and g) enlarged part of image f showing overlapped iFFT
and corresponding interplanar distance. h) SAED pattern of TeKCNIr showing periodic bright spots for monocrystalline Te. EDX elemental mapping of
i–n) TeKCNIr NRs for K, Te, N, C, O and o–t) TeKCNIr NSs HAADF, Te, O, N, K, and Te/K composite.
(TeNRs/NSs) using high polarity DMF (Figure S11, Supporting
Information).[11b]
However, the atomic distribution of the Ir sites
were clearly visible in the AC-HAADF-STEM images (Figure S12,
Supporting Information).
As expected, the diffuse reflectance UV-vis spectra of elemen-
tal Te show strong absorption in visible and near infra-red (NIR)
regions extended up to 1000 nm due to an ultrasmall band gap
(0.3 for bulk materials) (Figure 3a).[12,26]
CN displayed an intense
band with band tailing extended up to 450 nm due to direct band-
to-band transition from VB to CB (Figure 3a).[27]
On the other
hand, the KCN displayed increased visible absorption due to K
doping and extended conjugation mediated by CN2 functional
groups.[28]
It should be noted that KCN displayed improved NIR
absorption as well due to the increased electron density of car-
bon nitride, in line with previous reports on alkali metal-doped
carbon nitride.[29]
While the KCN-wrapped TeKCN shows inter-
mediate absorption due to combinational UV–vis absorption of
Te and KCN components.[30]
The IrSA decorated TeKCNIr cata-
lysts do not show any significant change due to the less populated
concentration of IrSA sites. The FTIR spectra of KCN and KCN-
containing composites demonstrated the presence of CN2 func-
tional groups (2166 cm−1
) along with heptazine ring stretch and
bending vibrations (Figure S13a, Supporting Information).[9]
Ra-
man spectra of KCN show a signature band ≈ 1609 cm−1
due to a
combination of D and G bands originating from in-plane and out-
of-plane vibration of sp2
hybridized C-N functionalities (Figure
S13b, Supporting Information).[31]
Elemental Te NRs/NSs show
an intense band at 123 cm−1
for A1 mode along with a small
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Figure 3. a) DR-UV–vis spectra, b) synchrotron-based WAXS spectra, inset 2D WAXS map of TeCNIr c) PL spectra at an excitation wavelength of 370 nm.
Excitation-Emission Matrix Spectroscopy (EEMS) map of d) CN, e) KCN, f) Te and g) TeKCN showing C K, N K, K L, and Te L edges. NEXAFS spectra of
materials in h) C K-edge i) N K-edge j) Te L-edge region. Color: CN (black), KCN (violet), KCNIr (green), Te (yellow), TeKCN (blue), TeKCNIr (red).
contribution of the E2 band which remains preserved after wrap-
ping KCN.[32]
Additional peaks at 354, 545, 985, and 1158 cm−1
were also observed for TeKCN deciphering the presence of the
TeO2 phase in the materials.[33]
In contrast to CN, the XRD pat-
tern of KCN shows intense peaks at 28.4°, 40.5°, 50.1°, 58.7°,
66.5°, and 74.1° for residual KSCN salt suggesting not all KSCN
was decomposed during the annealing process (Figure S13c,
After integration of Te and Ir SA
sites the XRD clearly demonstrates the presence of Te and
TeO2 peaks in the TeKCNIr catalyst (Figure S13d, Supporting
Information).[35]
The presence of TeO2 peak suggest surface oxi-
dation of Te nanostructures during synthesis steps. Synchrotron-
based wide-angle X-ray scattering (WAXS) 2D map and az-
imuthally integrated intensity versus q plot of CN and KCNIr
show an intense scattering ring originated from (002) plane with
a small contribution of (100), (006), and (004) planes (Figure
S14a,b, Supporting Information).[36]
After hybridization of Te
with KCN (TeKCN) and Ir SA, a small diﬀraction ring and pat-
tern were observed at 2.64 and 2.78 Å−1
Q values suggesting in-
tegration of Te in composites (Figure 3b and Figure S14c).[37]
No peak associated with nanometric or sub-nanometric Ir-related
species was observed substantiating the presence of Ir sites in
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atomic state. The charge carrier dynamics and emissive quench-
ing in CN-based materials were determined using steady-state
photoluminescence (ssPL) spectroscopy (Figure 3c). As expected,
PL spectra of CN displayed an intense broad band centered at
463 nm due to direct band-to-band emission quenching of charge
carriers in the 2D heptazine motif.[38]
A significant quenching
was observed for KCN which was associated with better charge
separation between alkali metal ion and CN framework.[39]
The
addition of Te NRs/NSs was found to further reduce PL inten-
sity substantiating efficient charge transfer between Te and KCN.
Interestingly, PL emission of TeKCNIr was almost completly
quenched which corroborates an effective charge quenching by
Ir atomic sites.
XPS survey scan spectra of CN, KCN and TeKCNIr depicted
the presence of all core and sub-core-level peaks corresponding
to constituting elements (Figures S15,S16, Supporting Informa-
tion) The C1s HR-XPS of KCN show characteristics peak for
sp2
hybridized C-(N)3 and N = C-N carbons for N-linked hep-
tazine network as present in CN framework (Figures S16,S17,
Additionally, a doublet for K2p3/2
and K2p1/2 components for doped K was also evidently visible.[41]
The N1s spectra of CN and KCN remain almost similar with
a slight increase in the intensity of 𝜋-𝜋* transition suggesting
improved conjugation in CN2 functionalized and K-doped car-
bon nitride.[42]
Apart from the uncondensed/residual N = C-O
and C = O peaks in O1s spectra, additional peaks at relatively
higher binding energies were associated with residual S-O incor-
porated from KSCN.[43]
KCNIr with Ir also demonstrated a sim-
ilar XPS pattern and no detectable signals for Ir were identified
due to low atomic concentrations (Figure S18, Supporting Infor-
mation). The KCN-wrapped Te NRs/NSs (TeKCN) do not show
any change in C1s, N1s, and O1s spectra except for additional
peaks for surface adsorbed Te-OH functionalities (Figure S19,
Supporting Information). The XPS in Te3d region displayed two
peak components corresponding to Te3d5/2 and Te3d3/2 peaks at
BE ≈575.9 and 586.4 eV respectively suggesting the presence of
Te in the elemental (Te0
) state.[44]
Conversely, TeKCNIr showed
intense Te3d5/2 and Te3d3/2 signals for the TeO2 chemical state at
a relatively higher BE value.[45]
The fact is that IrSA site decora-
tion was performed in the aqueous state which led to the surface
oxidation of Te NRs/NSs making Te/TeO2 core-shell structure.
The N1s XPS spectra of TeKCNIr were drastically changed due
to the contribution of N atoms from pyrrolidone capping agents
on the surface of TeNRs/NSs. Additionally, the Ir4f peaks (4f7/2
and 4f5/2) were also observed at BE values of 63.9 and 68.5 eV
corroborating the presence of Ir sites on the surface of TeKC-
NIr catalysts.[46]
TeKCNIrP prepared by using DMF as solvent
demonstrates intense TeO2 peak components because of a high
degree of oxidation as DMF removes surface capping agents from
Te NRs/NSs.
Near edge X-ray absorption fine structure (NEXAFS) spec-
troscopy using synchrotron-based soft X-ray was performed to
further investigate the chemical and structural changes in the
materials. Excitation-Emission Matrix Spectroscopy (EEMS) in
the energy range of 250 to 2000 eV to identify the constitutional
elemental edge demonstrates the presence of C K-edge, N K-edge
for CN while KCN displayed an additional K L-edge (Figure 3d,e)
due to K doping. Pristine Te NRs/NSs show a very faint Te M-
edge due to less populated transitions (Figure 3f). The presence
of an intense peak for C K-edge suggests dense PVP capping lig-
ands. The C K-edge NEXAFS spectra of CN and KCN revealed
strong 𝜋* resonance peaks at 284.4 eV for N = C-N carbons
with a small 𝜋*C = C contribution from adventitious carbons at
281.8 eV (Figure 3h).[47]
The 𝜎* region of the C K-edge showed
a broad band originating from 𝜎*N-C = N and 𝜎*C-N excitation.[48]
No distinguishable change in C K-edge spectra was observed af-
ter the integration of Te NRs/NSs. The N K-edge NEXAFS spec-
tra of CN exhibited displayed two 𝜋* resonance peaks at 399.2
and 402.1 eV arising from 𝜋*C–N = C and 𝜋*N–C3 transition of ter-
tiary N-bridged C6N7 network (Figure 3i).[49]
A broad band with
faint peaks due to 𝜎* transition of sp2
hybridized C-N and C-N
= C moieties was also observed substantiating a well-constituted
CN framework.[50]
Distinctly, KCN displayed an intense peak at
399.4 eV due to 𝜋* transition of cyanamide (CN2) and uncon-
densed N = C-O functionalities. Interestingly, these transitions
disappeared in TeKCNIr catalysts which might be due to sup-
pressed 𝜋* transition in NCN/N = C-O interacted Te nanostruc-
tures. Pristine Te NRs/NSs displayed a broad absorption band
assigned to Te M-edge in the energy range of 577.4-583.8 eV
(Figure 3j).[51]
This band was still visible in the TeKCN composite
however, the intensity was significantly reduced to a small con-
centration of Te in the composite.
To validate the presence of isolated Ir sites on the TeKCN
structure, AC-HAADF-STEM images of TeKCNIr were collected
which displayed bright spots for Ir species in the CN scaffold.
Most of the high Z-contrast Ir sites represented the presence of an
isolated atomic state, however, small regions with Ir aggregates
were also present in the material (Figure 4a–c). The presence of
Ir nanoclusters in AC-HAADF-STEM might be due to minor ag-
glomeration during the synthesis stage or exposure/damage of
samples to an electron beam (200 keV).[52]
Electron energy loss
spectroscopy (EELS) elemental mapping of TeKCNIr displayed
intense Te L-edge and O K-edge signals centered on the TeNRs
structure (Figure 4d). The presence of intense O K-edge signals
centered on Te nanorods demonstrated partial surface oxidation
and the presence of Te/TeO2 nanostructure. While C K-edge, N K-
edge and K L-edge signals were distributed around the Te/TeO2
suggesting successful wrapping of KCN (Figure 4d). The pres-
ence of KCN-wrapped structures was clearer in Te, K, C, and N
composite images. No signals corresponding to Ir were identified
due to dilute concentration.
The chemical and coordination structure of Ir species present
in the TeKCNIr catalyst was determined using X-ray absorp-
tion near-edge structure (XANES) spectroscopy collected using
synchrotron-based hard X-ray radiation. The Ir L3 XANES spec-
tra of TeKCNIr exhibited a sharp rising edge at 11 208 eV origi-
nating from the 2p→5d (t2g
5
→eg
0
) transition (Figure 4e).[53]
The
position of this edge was at higher energy than Ir foil and IrCl3
(Ir3+
) and coincided with the IrO2 edge suggesting the presence
of Ir sites as IrO2 (d56s0
state).[54]
Furthermore, the white line
intensity of TeKCNIr was significantly increased suggesting re-
duced electronic density on the Ir site due to partial charge trans-
fer to N atoms in the carbon nitride cavity.[55]
Contrarily, CNIr
does not show a significant enhancement in white line intensity
suggesting that KCN can more efficiently coordinate with the Ir
site.
To evaluate the coordination state and electronic environments
Fourier-transformed extended X-ray absorption fine structure
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Figure 4. AC-HAADF-STEM images of TeKCNIr at a) 5 nm scale bar showing bright spots for IrSA sites b) Enlarged area of the image a showing IrSA
sites distributed in CN matrix. c) at 5 nm scale showing another region with uniform Ir single atom distribution. d) EELS ADF image showing scanned
area and EELS elemental mapping for Te, O, N, K, C, RGB composite of Te (red), C (green), K (blue), RGB composite of Te (red), K (green), and N (blue).
e) Ir L3-edge XANES spectra of Ir foil, CNIrSA, IrCl3, IrO2, TeKCNIrP and TeKCNIr. f) FT-EXAFS spectra and fitting for Ir foil, IrO2, CNIr and TeKCNIr.
WT EXAFS map of g) Ir foil h) IrO2 i) CNIr j) TeKCNIr.
(FT-EXAFS) spectra of materials were acquired in Ir LIII-edge
region (Figure 3f; Figure S22, Supporting Information) The FT-
EXAFS spectra of metallic Ir foil displayed a sharp feature at 2.57
Å due to Ir-Ir scattering with another scattering at higher value
from distant atoms scattering (Figure 3f).[56]
For IrO2 powder the
first shell Ir-O scattering was observed at 1.62 Å while the sec-
ond shell Ir-O-Ir interaction was observed at R(Å) value ≈3.45
Å.[57]
On the other hand, CNIr and TeKCNIr exhibited only one
peak at 1.44 and 1.53 Å, respectively attributed to the Ir-O/Ir-N
scattering.[20]
Very weak scattering for Ir-Ir or Ir-O-Ir deciphering
most of the Ir was present in an isolated state and contribution of
agglomerates Ir sites was negligible. To further gain insight into
coordination and bonding patterns EXAFS data fitting was exe-
cuted and obtained values were compared with DFT simulated
models (Tables S1,S2 and Figure S23, Supporting Information).
The EXAFS data fitting exhibited the Ir─O bond length of 1.80
and 1.84 Å for CNIrSA and TeKCNIr while Ir-N bond lengths
were calculated to be 2.05 and 2.11 Å, respectively. The obtained
values closely matched with the DFT constructed model of CNIr
and KCNIr and suggested Ir sites were mainly located on KCN
surface (Figure S23 and Table S2, Supporting Information). The
obtained values were relatively smaller than Ir-Ir (2.71 Å) and Ir-
O (1.99 Å) bond lengths in Ir foil and IrO2 nanoparticles suggest-
ing Ir was distinctly coordinated with N and O atoms. The Ir-O
and Ir-N coordination number (C.N.) of TeKCNIr was calculated
to be 1.92 and 1.73 suggesting IrN2O2 (C.N.−3.65) sites were en-
tirely different than IrO2 NPs (C.N.−6) and Ir metal (C.N.−12).
CNIr demonstrates Ir-N and Ir-O C.N. of 1.97 and 1.83 with a
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Figure 5. a) TRPL spectra of CN and TeKCNIr b) Photocurrent density versus time (J-t) plot during the light on-off cycle c) Photocatalytic H2 evolution in
the presence/absence of TEOA using CN, CNIr, KCN, TeKCN, TeKCNIr after 24 h d) Photocatalytic glycerol conversion and e) glyceraldehyde yield and
f) glyceraldehyde selectivity as a function of time under AM1.5G irradiation using Te, TeKCN5, TeKCN10, TeKCN15, CNIr, CN, and TeKCNIr.
total C.N. value of 3.80 which was slightly higher than TeKC-
NIr (3.65) indicating an undercoordinated state of IrN2O2 sites
in TeKCNIrSA. These undercoordinated IrN2O2 sites can inter-
act strongly with the adsorbate molecule for accelerated reaction.
TeCNIrP catalysts also demonstrate similar features as TeKCNIr
except the presence of a small amount of Ir-Ir scattering which
suggests DMF promotes agglomeration of Ir sites.
Wavelet transform (WT) EXAFS spectra of Ir foil, IrO2, CNIr
and TeKCNIr were also plotted to understand the electronic struc-
ture of Ir species (Figure 4g–j). The WT-EXFAS spectra of Ir foil
showed a bright zone at K = 12.90 Å−1
and R = 2.54 Å originat-
ing from Ir-Ir scattering while a faint zone at K = 14.17 Å−1
and
R = 5.02 Å occurs from scattering from Ir atoms in second coor-
dination shell (Figure 4g).[58]
For IrO2, scattering from Ir-O first
shell coordination gives a broad contour zone at K = 8.19 Å−1
and
R = 1.64 Å while Ir-O-Ir second shell scattering was observed at
K = 13.59 Å−1
and R = 3.46 Å (Figure 4h).[59]
CNIr displayed a
sharp zone at K = 5.38 Å−1
and R = 1.60 Å while K = 5.38 Å−1
and R = 1.61 Å assigned to Ir-N/Ir = O first shell scattering.[60]
The obtained R-space values for CNIr and TeKCNIr were close to
IrO2 while K space values were significantly different suggesting
IrO2 entities have distinct electronic environments due to the co-
ordination with N atoms in the C6N7 cavity. Further, the absence
of any scattering zone at a higher R space value suggests that Ir
sites were atomically isolated.
2.2. Photophysical and Photoelectrochemical Performance
The charge recombination dynamics and lifetime of photogener-
ated charge carriers were determined using time-resolved pho-
toluminescence (TRPL) spectroscopy using a 370 nm excitation
pulsed laser (Figure 5a and Table S3, Supporting Information).
The PL lifetime decay curve of CN and TeKCNIr was best fit-
ted tri-exponentially which agreed well with reported literature
on CN-based materials.[61]
The spectra were fitted using the fol-
lowing equations.
I (t) = A1e−t∕𝜏1
+ A2e−t∕𝜏2
+ A3e−t∕𝜏3
(1)
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where, A1, A2 and A3 represent the normalized amplitudes
contribution of decaying components while 𝜏1, 𝜏2 and 𝜏3 are in-
dividual lifetime of each component, respectively. The calculated
lifetime value and fractional contributions of decay components
are summarized in Table S3 (Supporting Information). The N-
linked C6N7 moieties in CN scaffolds contain sp3
and sp2
hy-
bridized C-N resulting in the formation of 𝜎 and 𝜎* molecular
orbital (MO) and 𝜋 and 𝜋* MO. Furthermore, the lone pairs on
secondary nitrogen (:N-C2) can also participate in conjugation
forming hybrid 𝜋+LP bonding and antibonding orbitals. The ex-
tended 𝜋+LP conjugated network results in the formation of con-
duction and valence bonds with low bandgap.[38,61a]
The direct
band-to-band recombination of (𝜎*→𝜎 and 𝜋*→(LP+𝜋)) photo-
generated carriers give rise to the first two shorter lifetime values.
The third lifetime with a relatively longer recombination time
originated from the 𝜎*→𝜋* non-radiative intersystem crossing
(ISC) and subsequent 𝜋*→(LP+𝜋) radiative recombination.[62]
The presence of defect states and shallow traps due to uncon-
densed units also contributes to the third lifetime of carbon
nitride.[63]
Interestingly, the TeKCNIr heterostructure demon-
strated decreased lifetime for all three components attributed to
decreased radiative recombination processes. The decreased life-
time might be due to the presence of sub-energy level defects and
interfacial charge transfer.[64]
Additionally, the average lifetime
(𝜏avg) which represents overall recombination was calculated us-
ing the following equation.
𝜏avg = (A1𝜏1
2
+ A2𝜏2
2
+ A3𝜏3
2
)∕(A1𝜏1 + A2𝜏2 + A3𝜏3) (2)
The average lifetime for CN was calculated to be 8.47 ns while
the value of cumulative lifetime for TeKCNIr was found to be
7.04 ns. The decreased lifetime suggests decreased radiative re-
combination processes and indicates better charge transfer in
IrSA decorated heterostructure.
To discern the enhanced visible-light-induced charge genera-
tion, the photoelectrochemical (PEC) performance was evaluated
for the TeKCNIr (Figure 5b and section 3.0 in Supporting In-
formation). The PEC experiments were carried out in a three-
electrode set-up using fluorine-doped tin oxide (FTO) as a con-
ductive substrate followed by deposition of catalysts and used
as a photoanode. While Ag/AgCl and Pt were used as reference
and counter electrodes. The photocurrent density of the materi-
als with respect to time (J-t curve) was measured at 0.6 V ver-
sus Ag/AgCl (1.23 V vs RHE (reversible hydrogen electrode);
water oxidation potential) (Figure 5b). The photoanode was ir-
radiated under AM1.5G solar irradiation (100 mW cm−2
) and
light On-Off cycles were performed to validate the true origin
of photocurrent from water splitting. Evidently, K doping in the
CN lattice and CN2 functionalization improved the photocatalytic
charge generation in the materials reaching a current density of
0.14 mA cm−2
compared to pristine CN (0.10 mA cm−2
). After the
hybridization of KCN with Te nanostructures, TeKCN displayed
further improvement in photocurrent density (0.20 mA cm−2
)
deciphering better charge generation and separation in the ma-
terials. When TeKCNIr with isolated Ir SA sites was used as a
catalyst, the maximum current density of 0.24 mA cm−2
was
achieved which was attributed to efficient charge capture by
charge-deficient Ir species. The electrochemical impedance spec-
troscopy (EIS) Nyquist plot demonstrates a smaller semicircle arc
for TeKCNIr compared to pristine KCN and CN suggesting lower
charge transfer resistance (Figure S24, Supporting Information).
The Randle circuit obtained by fitting of EIS curve demonstrates
decreased charge transport resistance in TeKCNIr (9.57 Ω) com-
pared to pristine CN (11.81 Ω) deciphering better migration of
charge carrier in developed heterostructure (Table S4, Supporting
Information). Furthermore, the catalytic performance of materi-
als toward hydrogen evolution without and with triethanolamine
(TEOA) as a sacrificial donor was also tested to confirm improved
charge separation and catalytic performance (Figure 5c). Pris-
tine CN displayed negligible (7.5 μmol g−1
cat) H2 evolution for
pure water splitting after 24 h irradiation with only slight im-
provement (14.0 μmol g−1
cat) in the presence of a TEOA sacrifi-
cial donor. In contrast, CNIr exhibited improved performance in
the presence of TEOA reaching a total H2 evolution of 27.7 and
132.4 μmol g−1
cat without and with sacrificial donor, respectively.
In contrast, KCN itself displayed a higher H2 evolution rate reach-
ing a maximum yield of 90 and 361 μmol g−1
cat without and with
TEOA. Despite a higher photocurrent density, TeKCN was unable
to further boost the H2 evolution rate (19.5 and 339.9 μmol g−1
cat
without and with TEOA) which could be due to the lack of plenty
of active sites on Te/KCN interface. Interestingly, TeKCNIr het-
erostructure with Ir atomic sites show a significant improvement
in H2 evolution rate 373.2 and 1595.3 μmol g−1
cat with and with-
out TEOA which was ≈50 and 114 times higher than that of CN.
2.3. Photocatalytic Conversion of Glycerol to Glyceraldehyde
Encouraged by these results, we have explored the scope of TeKC-
NIr for the selective oxidation of glycol to glyceraldehyde in aque-
ous and photocatalytic conditions (section 4.2 in Supporting In-
formation). The product was analyzed by HPLC and 1
H NMR
which clearly showed that TeKCNIr promotes facile conversion
of glycerol to glyceraldehyde (Figures S25–27, Supporting Infor-
mation). Time-dependent glycerol conversion demonstrated that
pure Te NRs/NSs were almost inactive for the glycerol conver-
sion while CN exhibited sluggish performance with a maximum
conversion rate of 9.7% after 24 h under AM1.5G irradiation
(Figure 5d). The glyceraldehyde yield using CN as a photocata-
lyst was found to be 3.2% after 24 h (Figure 5e). CNIr with a dec-
oration of isolated IrN2O2 site improved the glycerol conversion
(17.9%) and glyceraldehyde yield (10.2%) which undeniably de-
picts isolated Ir site increased the charge carrier separation and
glycerol conversion. Interestingly, TeKCN5 containing KCN and
5% TeNRs/NSs demonstrated a significant improvement in glyc-
erol conversion (37.5%) and glyceraldehyde yield (19.3%). The
enhanced activity for TeKCN was attributed to better charge sepa-
ration in Te and KCN heterostructure and was consistent with the
photocurrent density profile. TeKCN10 shows a slight improve-
ment in glycerol conversion (39.4%) and glyceraldehyde yield
(21.0%). Further, an increase in Te content to 15% does not im-
prove the catalytic performance of TeKCN heterostructures sug-
gesting TeKCN10 was the optimum catalyst. The glycerol conver-
sion using TeKCN15 was found to be 31% after 24 h. After the
decoration of IrN2O2 isolated sites on TeKCN10 heterostructure
(TeKCNIr), the glycerol conversion rate was slightly improved
reaching a value of 45.6% while the glyceraldehyde yield was sig-
nificantly increased (28%). The increased glyceraldehyde yield
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Figure 6. Photocatalytic a) glycerol conversion b) glyceraldehyde yield and c) glyceraldehyde selectivity under AM1.5G irradiation and 365, 400, 450,
500, 550, 600, and 650 nm LEDs using TeKCNIr catalyst. d) glycerol conversion e) glyceraldehyde yield and f) glyceraldehyde selectivity after 24 h in the
presence of BQ, IPA and EDTA as O2
•−, •OH and h+ scavengers respectively (AM1.5G irradiation). EPR spectra of reaction product using under dark
and after 5, 10 min irradiation under AM1.5G irradiation using g) TEMPO as photogenerated hole trapping agent and DMPO as h) superoxide radical
i) hydroxyl radical spin labeling agent.
suggests that isolated Ir sites promote selective oxidation of glyc-
erol to glyceraldehyde (Figure 5e). The calculated glyceraldehyde
selectivity for CN was 32.5% while glyceraldehyde selectivity for
CNIr was found to be 57% which indicates the role of Ir SA
sites in improved catalytic selectivity (Figure 5f). The glyceralde-
hyde selectivity of TeKCN5, TeKCN10 and TeKCN15 was 51.6,
53.2 and 31.0% respectively suggesting TeKCN10 heterostructure
itself has better selectivity. When TeKCNIr was used as a cata-
lyst the highest observed selectivity was 61.6%. These observa-
tions indicate that the decoration of Ir SA sites provides active
centers for improved selectivity. After 24 h reaction, glyceric acid
was identified as a major by-product with an overall selectivity of
≈19.8% while other possible products such as DHA were neg-
ligible (Figure S26, Supporting Information). The TeKCNIr se-
lectivity was higher compared to reported state-of-the-art cata-
lysts while most of the catalysts demonstrate DHA as a dominat-
ing product (Table S5, Supporting Information). Furthermore,
the evolved gaseous products were also analyzed which demon-
strated H2 as a major product with a trace of CO2. After 24 h
irradiation, the yield of H2 was found to be 214.2 μmol g−1
cat.
While CO2 was significantly low and was not quantified. The ob-
tained results demonstrate glycerol provides protons for the H2
generation.
To understand the role of specific energy of light on the
photocatalytic activity of TeKCNIr, wavelength-dependent glyc-
erol conversion and glyceraldehyde yield were also calculated. It
can be seen from Figure 6a,b that solar-simulated AM1.5G ir-
radiation displayed the highest glycerol conversion and glycer-
aldehyde yield. Among various wavelength LEDs (Power den-
sity 1.1 mW cm−2
), 400 nm LED displayed the highest glyc-
erol conversion of 9.7% with a yield of glyceraldehyde equiv-
alent to 8.4%. On the other hand, at 450 nm wavelength, the
glycerol conversion and glyceraldehyde selectivity were 6.3 and
5.6%, respectively. After that, the glycerol conversion gradually
depletes as the wavelength of light increases which was due to
less energetic electron hole pairs. This suggests that KCN with an
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absorption edge of 450 nm generates electron-hole pairs while
Te nanostructures facilitate charge separation which is consis-
tent with previous literature.[12]
Interestingly, monochromatic ra-
diation increases the selectivity of glyceraldehyde compared to
AM1.5G light (Figure 6c). The glyceraldehyde selectivity using
400, 450, and 365 nm LED was found to be 87.1, 88.6, and
75.2%, respectively. The unusually high selectivity (88.6%) at
450 nm wavelength light attributed to the predominant excita-
tion of charge carriers in KCN structure and has been further
discussed in the following sections.
To discern the role of reactive species responsible for the se-
lective oxidation of glycerol to glyceraldehyde, the reaction was
carried out using benzoquinone (BQ), isopropyl alcohol (IPA)
and ethylene diamine tetraacetic acid (EDTA) as superoxide ra-
dial (O2
•−
), hydroxyl radical (•OH) and hole (h+
) scavengers, re-
spectively (Figure 6d–f). Compared to the reaction without any
scavenger (45.6%), using IPA and EDTA the glycerol conversion
was slightly decreased to 42.4 and 40.1%, respectively (Figure 6d).
When BQ was used as a superoxide radial (O2
•−
) scavenger the
glycerol conversion was decreased to 30.3%. The glyceraldehyde
yield and selectivity also follow a similar pattern. A significant
drop in the selectivity using BQ suggests O2
•−
was the main re-
active oxygen species (ROS) for the oxidation of glycerol. How-
ever, •OH and h+
also participate in the reaction, but their relative
contribution remains low compared to O2
•−
radicals. These find-
ings are well in agreement with previous literature which demon-
strates that O2
•−
radicals promote selective oxidation of organic
substrate.[65]
The presence of glyceric acid as a by-product sug-
gests overoxidation of glyceraldehyde can lead to glyceric acid.
To understand the influence of oxygen on the reaction, glycerol
oxidation in a stainless pressure reactor equipped with a quartz
window and 8 bar O2 (Figure S28, Supporting Information). Af-
ter 24 h simulated solar light irradiation, the glycerol conversion
was found to be 66.3% while the glyceraldehyde selectivity de-
creased to 26.6% (Figure S29, Supporting Information). The in-
creased glycerol conversion indicates the reduction of O2 to O2
•−
radical anion followed by glycerol oxidation was a more favorable
reaction route. However, decreased glyceraldehyde selectivity also
substantiates over-oxidation of glyceraldehyde under pressurized
O2 conditions.
To further validate the generation of ROS and electron-hole
pairs, electron paramagnetic resonance (EPR) spectroscopy was
performed using spin trap agents under dark and light con-
ditions (Figure 6g–i). The EPR spectra acquired using 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) and TeKCNIr catalysts
under dark conditions exhibited intense triplet with an intensity
of 1:1:1 which was signature signals for TEMPO (Figure 6g).[66]
After visible light irradiation, the signal intensity for TEMPO
was decreased due to the trapping of photogenerated electrons
by TEMPO followed by the formation of EPR silent hydroxyl
amine of TEMPOH. The reduced signal intensity under visible
light irradiation suggests populated charge carrier generation in
the TeKCNIr heterostructure. Furthermore, the EPR experiment
carried out using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a
spin trap showed well resolved quartet (1:1:1:1) due to the for-
mation of DMPO-O2
•-
species (Figure 6h). The signal intensity
was increased after increasing the irradiation time (5 and 10 min)
suggesting continuous generation of O2
•−
species under photo-
catalytic conditions. Additionally, a quartet with 1:2:2:1 intensity
was also observed deciphering the formation of hydroxyl (•OH)
radical during the visible light irradiation (Figure 6i). These ob-
servations suggest a direct role of photogenerated electrons, su-
peroxide radical O2
•-
and hydroxyl radicals (•OH) in the transfor-
mation of glycerol to glyceraldehyde. The glycerol oxidation on
TeKCNIr proceeds via two electrons and two protons abstraction
from glycerol at IrN2O2 sites on VB followed by reduction of in-
termediates at the CB. So, reactions at both CB and VB contribute
to product formation. The addition of scavengers reduces the re-
action rate either at CB or VB due to competition for active sites,
which interferes with charge transfer and proton from glycerol
molecule and another dominant reaction pathway might be pre-
ferred which reduces the product selectivity.
Temperature programmed desorption (TPD) using probe
molecule titration was implemented to understand the nature
and counts of Lewis/Brønsted acid and base sites (Figures
S30,S31, Supporting Information). TPD using CO2 (a weak Lewis
acid) provides information on Lewis base sites while pyridine
being a strong base will titrate all types of acidic sites giving
a total acidic site count. Sterically hindered molecule DTBP
(2,6-di-tert-butylpyridine) can only titrate Brønsted Acid sites.[67]
The difference between total and Brønsted acid sites provides
Lewis acid count (Figure S32, Supporting Information). CO2-
TPD of CN shows a strong desorption peak at 168 °C due to
the presence of secondary: N-C2 nitrogen of heptazine (C6N7)
ring system and uncondensed: NH/NH2 (Figure S30a, Support-
ing Information).[68]
The doping of K+
does not change the CO2
desorption profile (165 °C) suggesting the nature of basic sites
and heptazine ring coordination pattern remain almost the same
in KCN. The CO2-TPD of TeKCN shows a parent KCN peak at
161 °C along with a weak shoulder peak at 95 °C. The weak des-
orption peak at 95 °C might arise due to surface adsorb H2O
molecules (Lewis base) or residual polyvinylpyrrolidone (PVP) on
TeNRs/NSs surface. After the decoration of IrN2O2 SA sites, the
CO2 desorption peak appeared slightly at higher temperatures
(higher basic character). The observed effect might be due to in-
creased charge density on the C6N7 motif from Ir metal to lig-
and charge transfer (MLCT) and stronger CO2 adsorption on IrO2
sites.[69]
Pyridine-TPD of CN, KCN and TeKCN show similar des-
orption profiles with a major desorption peak at 83, 88 and 86
°C (Figure S30b, Supporting Information). Though CN structure
does not have any specific acidic sites, chemisorbed O-H groups
on CN can behave as Brønsted acid sites.[70]
The low desorption
temperature for pyridine also supports the presence of weakly
bonded OH on the CN/KCN’s surface. TPD peak at higher tem-
peratures (175–180 °C) was attributed to strongly bonded -OH
produced from incomplete condensation of CNs. TeKCNIr shows
three weak desorption peaks at 64, 111 and 149 °C. Consider-
ing the very weak desorption profile (acid strength) of 64 and
111 °C peaks from chemically adsorbed O-H groups, it can be
inferred that the acidic sites should not govern the catalytic ac-
tivity of TeKCNIr. The peak at a relatively high temperature (149
°C) was attributed to the desorption of pyridine from Ir-O–OH
sites formed by hydrolysis of surface adsorbed water. DTBP (2,6-
di-tert-butylpyridine) TPD also reveals the presence of a similar
desorption profile confirming the presence of Brønsted acid sites
on TeKCNIr (Figure S31, Supporting Information). The active
site count analysis shows TeKCNIr possesses the highest num-
bers of basic sites (1.0 μmol g−1
cat). As expected, Lewis acid count
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Figure 7. a) Band diagram of Te, KCN before and after contact showing the charge flow and plausible mechanism of glycerol oxidation. b) optimized
geometry of TeKCN, c) charge density difference iso-surface of TeKCN and d) optimized structures of TeKCNIr.
remains almost constant in all materials and therefore cannot be
accounted as the active site in catalysis.
The visible light absorption profile and bandgap energies (Eg)
of the materials were determined using the Kubelka-Munk func-
tion (Figure S33, Supporting Information).[71]
The optical band
gap of Te NRs/NSs, CN, KCN, KCNIr, TeKCN, and TeKCNIr
was calculated to be 0.32, 2.72, 2.62, 2.66, 1.89, and 1.94 eV
respectively. Furthermore, Mott-Schottky analysis of Te, KCN,
TeKCN and TeKCNIr demonstrated the flat band potential (Efb)
value −0.14, −0.47, −0.31, and −0.52 V respectively versus NHE
at pH-0 (Figure S34, Supporting Information). Considering the
Fermi level lies close to the CB in n-type materials, these val-
ues represent the position of the CB. The position of Fermi level
with respect to the vacuum level (Evac) was determined using ul-
traviolet photoelectron spectroscopy (UPS) work function (WF)
spectroscopy (Figure S35, Supporting Information). The work
function (ϕ) value was estimated by subtracting the cut-off en-
ergy of secondary electrons (Ecut-off) from the energy of the He-II
UV source (21.21 eV; WF (ϕ) = 21.21−Ecut-off) (See SI for more
detail).[72]
From the point of inflections, the Ecut-off energies of CN,
KCN, TeKCN and TeKCNIr were calculated to be 16.30, 16.33,
16.35, and 16.36 eV and the corresponding WF values w.r.t to
Evac were 4.91, 4.88, 4.86, and 4.85 eV respectively. Additionally,
the VB) position calculated using XPS valance band (XPS-VB)
spectra for CN, KCN, TeKCN and TeKCNIr was 1.98, 1.70, 1.48,
and 1.46 eV below the Fermi level (Figure S36, Supporting Infor-
mation) Based on band energies calculations, the band diagrams
of materials were constructed (Figure S37, Supporting Informa-
tion). The TeNRs/NSs due to the low band gap can absorb in vis-
ible to NIR region. Due to the intrinsic p-type nature of bulk Te,
it makes a p-n heterojunction with n-type KCN (Figure 7).[73]
Af-
ter contact, electrons from the conduction band of n-type KCN
flow to Te during the Fermi level equilibration and a Type-II (stag-
gered) heterojunction was established. In TeKCN heterojunction,
after absorption of visible light, the photogenerated electrons on
Te migrate to KCN and resulting electrons reduced O2 to O2
•−
.
The resulting O2
•−
can either directly participate in glycerol ox-
idation or are transformed to •OH radicals which oxidize glyc-
erol. It can be seen from the VB position of KCN that photogen-
erated holes do not possess the required oxidation potential to
generate •OH from water (2.32 V vs NHE at pH-0).[74]
Therefore
observed •OH radicals in the EPR originated from the indirect
route via reduction of O2. However, due to the low oxidation po-
tential of glycerol (0.004 V vs NHE at pH-0), the holes in the VB
of KCN can directly participate in glycerol oxidation.[75]
The iso-
lated IrN2O2 SA sites improve the catalytic activity due to their
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excellent hole-capturing ability thus reducing the recombination
losses. Additionally, as found from DFT analysis, IrN2O2 sites
also work as active sites to bind with glycerol molecules and fa-
cilitate its selective oxidation. The stepwise transfer of two holes
from IrN2O2 sites to surface adsorbed glycerol molecule followed
by proton abstraction and reduction of intermediates at the CB
led to the generation of glyceraldehyde. The high density of Lewis
base site on TeKCNIr also promotes facile electron transfer and
reduction of intermediate therefore improving the product yield.
The observed high selectivity (88.6%) at 450 nm for TeKCNIr het-
erostructure can be explained based on populated charge gener-
ation in KCN at lower wavelength. At higher wavelength only Te
NRs/NSs can be excited, and a significant charge loss takes place
at the interface. Furthermore, generated less energetic holes re-
mained confined to Te structure thus holes are unavailable to
IrN2O2 sites. However, when KCN was excited at a lower wave-
length, the photogenerated holes can be efficiently captured by
IrN2O2 sites which facilitates selective oxidation of adsorbed glyc-
erol molecules.
To understand the structure of materials and charge transfer
mechanism in TeKCNIr heterostructure, density functional the-
ory (DFT) was performed using Vienna ab initio simulation pack-
age (VASP). A single layer 2×2×1 supercell of CN with 32 nitro-
gen and 24 carbon atoms was considered with a vacuum layer of
20 Å and its optimized geometry is shown in Figure S38a (Sup-
porting Information). After optimizing the bulk structure of Te
structure, its (001) surface is generated and optimized. 3×3×1
supercell of Te surface with 54 atoms is considered to match the
size of the g-C3N4 supercell and the optimized structure of the Te
surface is shown in Figure S38b (Supporting Information) Opti-
mized cell parameters of CN (13.314 Å) and Te surface (12.767
Å) were found to have a mismatch of 4.27%. CN adsorbed on
Te surface has been optimized and reported in Figure S39a (Sup-
porting Information). Charge density difference iso-surface for g-
C3N4-Te system indicates no significant charge transfer between
the g-C3N4 and Te (Figure S39b, Supporting Information). On the
other hand, charge density difference iso-surface for TeKCN with
one K atom placed between the CN and Te shows a significant
electron transfer to the C3N4 surface indicating the role of K as
a bridge between the Te and C3N4 surfaces (Figure 7b,c). Planar
averaged electrostatic potential (Φvacuum) along the normal direc-
tion to the surface has been calculated for both CN and TeKCN
and the work function was calculated as (eΦvacuum – μFermi) (Figure
S40, Supporting Information). The calculated work function for
CN was found to be 6.15 eV whereas it is 4.59 eV for the Te-K-
C3N4. These results suggest facile charge transfer from Te to CNs
surface.
Furthermore, IrO2 unit were adsorbed over the CN, KCN,
TeCN and TeKCN surfaces with two different configurations. In
the first configuration, IrO2 unit is considered to be linear with Ir
atom interacting with nitrogen atoms of C3N4 and O atoms are
placed above and below the C3N4 sheet. While in second con-
figuration, both the O atoms of IrO2 are on the same side of
CNs sheet (Figure 7; Figure S23e,f, Table S2, Supporting Infor-
mation). From the optimized structures, the second configura-
tion was found to be the minimum energy structure. It can be
seen from the optimized structure that in TeKCNIr, IrO2 facing
toward the surface can potentially interact with glycerol molecule
during oxidation reaction.
3. Conclusion
We have designed an efficient Te/TeO2 core-shell structure dec-
orated with K-doped carbon nitride (KCN) van der Waal hetero-
junction by a simple electrostatic interaction approach. The dec-
oration of Ir single atom sites on TeKCN heterojunction by hy-
drolysis decomposition improved the catalytic conversion of glyc-
erol to glyceraldehyde under solar-simulated light without any
sacrificial donor. The catalytic selectivity for glyceraldehyde for-
mation can reach as high as 88% under 450 nm irradiation.
AC-HAADF-STEM and WAXS analysis reveal isolated Ir sin-
gle atoms decorated on KCN 2D structure. Synchrotron-based
XANES and EXAFS analysis validate the presence of underco-
ordinated IrN2O2 sites embedded in the CN framework. Due to
the presence of low bandgap Te nanostructure, a better charge
separation at the Te/KCN nano-heterointerface was achieved re-
sulting in improved catalytic performance. The IrN2O2 isolated
sites improve the catalytic selectivity for glyceraldehyde forma-
tion. Based on scavenger and EPR studies, superoxide radicals
and hydroxyl radicals were found to be the main reactive species
promoting the selective oxidation of glycerol. The findings pre-
sented in the manuscript will galvanize studies to develop active
and resilient SACs for the selective transformation of low-value
products to commodity chemicals.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors would like to thank the University of Calgary’s CFREF for finan-
cial assistance. Drs. Natalie Hamada and Brian Langelier at the Canadian
Centre for Electron Microscopy (CCEM), McMaster University are kindly
acknowledged for HR-TEM, AC-HAADF, and EELS analysis on the sam-
ples. Part of the research described in this paper was performed at the
Canadian Light Source (Project: 35G12344), a national research facility of
the University of Saskatchewan, which is supported by the Canada Founda-
tion for Innovation (CFI), the Natural Sciences and Engineering Research
Council (NSERC), the National Research Council (NRC), the Canadian In-
stitutes of Health Research (CIHR), the Government of Saskatchewan, and
the University of Saskatchewan. Drs. Ning Chen, Beatriz Diaz-Moreno, Jay
Dynes, Tom Regier, Zachary Arthur and Adam Leontowich are kindly ac-
knowledged for helping in hard/soft X-ray and WAXS analysis.
Conflict of Interest
The authors declare no conflict of interest.
Author Contributions
P.K. and J.W. contributed equally to this work. P.K. and A.A. conceived
the research, synthesized, and characterized the catalyst, evaluated part
of the catalytic performance, interpreted the results, and wrote the initial
draft of the manuscript. J.W. performed photocatalytic reactions and an-
alyzed the reaction product. S.R. conducted spectroscopic characteriza-
tions and chemical analysis on the catalysts and edited the manuscript. V.J.
and S.R. conducted the TPD measurements and analysis on the catalysts.
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X.W. helped with EXAFS analysis. H.H., K.K., D.T. helped in the catalyst’s
synthesis, characterization, and performance evaluation. S.K. performed
DFT analysis and compiled results. P.B., P.M.A., J.H. M.G.K. and M.A. su-
pervised the research and edited the manuscript. All co-authors read and
approved the final version of the manuscript.
Data Availability Statement
The data that support the findings of this study are available from the cor-
responding author upon reasonable request.
Keywords
doped carbon nitride, glycerol conversion, photocatalysis, photooxidation,
single atom catalysts
Received: November 4, 2023
Revised: February 24, 2024
Published online:
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